Normally, copper sulfide ore contains a small amount of silver. The silver is recovered in a smelting process by a common method that processes copper sulfide ore at a high temperature exceeding 1000° C.: The copper sulfide ore containing silver is processed at a high temperature exceeding 1000° C. together with iron sulfide and silicate ore so as to form Cu2S called matte, and slag composed essentially of iron oxide and silicate and containing impurities. The matte is reduced at a high temperature into low-purity metal copper called blister copper. The blister copper is then refined through electrolysis into metal copper having a purity of 99.99% or more.
Silver contained in copper sulfide ore migrates in company with copper in the production process of metal copper. In the electrolysis, the silver is recovered with other noble metals in the copper electrolytic precipitate, which is processed as follows: The precipitate is oxidized at a high temperature in a dry furnace to separate crude silver containing noble metals from the slag containing lead oxide. A silver anode is made from the crude silver and is electrolyzed to recover high-purity silver. The process is disclosed in Japanese Unexamined Patent Application Publication No. 2001-316736.
A disadvantage of this method is that a high temperature exceeding 1000° C. is required. Another disadvantage is that the process to recover silver requires many steps because silver is a byproduct of metal copper production.
Hydrometallurgy is a countermeasure that overcomes these disadvantages, as is described in co-pending Japanese Patent Application No. 2008-084597, which has not been published yet. The method of recovering copper and gold from ore described in this application comprises: (1) a copper leaching step of preparing a leaching solution containing cuprous ions and cupric ions by adding raw material sulfide ore to a first acidic solution containing alkali or alkaline earth metal chloride and bromide and copper and iron chlorides or bromides, and blowing air into the acidic solution under atmospheric pressure at a temperature below the boiling point of the solvent at least for a period of time; (2) a solid-liquid separation step of separating the leached raw material through solid-liquid separation; (3) an air-oxidizing step of blowing air into the solution after the solid-liquid separation to oxidize at least part of cuprous ions to cupric ions, to oxidize iron leached out during the copper leaching step, and to coprecipitate impurities leached out of the raw material during the copper leaching step, followed by precipitation separation; (4) a copper extraction step of extracting the copper through a process such as solvent extraction from the solution after the precipitation separation of the air-oxidizing step; (5) a gold recovery step of leaching gold from the residue separated in the solid-liquid separation step by adding the residue to a second acidic solution containing alkali or alkaline earth metal chloride and bromide and copper and iron chlorides or bromides and blowing air into the solution under atmospheric pressure at a temperature below the boiling point of solvent in the presence of iron.
The method can recover copper and gold through a leaching process at a high leaching ratio for copper sulfide ore in a chloride solution bath using only air with no special oxidizing agent. In this regard, leaching of copper in a chloride solution bath is advantageous compared to that in a sulfate solution bath. In such a case, silver is also eluted in the leaching solution.
Although the dissolution of the silver is not described in the application, silver is dissolved in the leaching process in the chloride or bromide bath containing copper and iron, so that the solution after extraction contains silver in addition to copper. Accordingly, it is desirable to recover the silver dissolved in the solution after extraction.
Japanese Patent 2857930 discloses a method of recovering silver from a chloride bath using mercury amalgam. Unfortunately, this method is not practical because of high toxicity of mercury. Although a method by solvent extraction is conceivable, this is not efficient because the distribution ratio is approximately one at a chlorine concentration of about 6 mol/L.